1. Field of the Invention
The present invention relates to a recording apparatus for optical data, more particularly to a recording apparatus for optical data, which uses limiter circuit to attenuate a reference voltage to minimize the influence of writing power to servo signal.
2. Description of Prior Art
The optical pick-up head in recording apparatus needs several servo control mechanism during the reading or recording optical data. For example, the laser of the pick-up head requires focusing control to control the focus point in vertical direction of optical disk and tracking control to control the focus point in radial direction of optical disk. Therefore, the laser can be precisely focused along the spiral track of the optical disk by feedback control of the servo controller.
FIG. 1 shows a schematic diagram of recording apparatus for optical disk.
To provide servo signal for error detection and servo control, a semi-cylinder lens 14 is placed in front of a photo detection IC 16 along the optical path. The semi-cylinder lens 14 provides different depth of focus along a vertical direction and a horizontal direction. In case that the recording apparatus for optical disk uses quad detector to detect optical signal, the quad detector will sense signal of different spiral shapes when the laser is moved in depth or radial direction.
When the laser has correct focusing in vertical direction, the laser will form a circle spot on the photo detection IC 16. When the laser has incorrect focusing in vertical direction, the laser will form spiral spot of different slopes on the photo detection IC 16. The spiral spot of different slopes can be used to create focus error (FE) signal. FIG. 2 shows a schematic diagram of FE signal generator. An FE signal is obtained by the arithmetic operation (A+C)−(B+D) and is output at an output end Vo.
Moreover, the optical disk generally has pre-grooves thereon. The laser beams reflected from the pre-grooves have asymmetric shape when the laser is not focused on center of track. The servo controller can produce track error (TE) signal according to the asymmetric signal reflected from the pre-grooves.
FIG. 3 shows a prior art push-pull circuit for processing the optical signals, wherein the push-pull signal (A+C)−(B+D) is output at Vo. The wobble signal of the optical disk can also be resolved from the push-pull signal. To enhance the resolution of the optical signals, as shown in FIG. 1, an optical grating 12 is provided to divide the optical beams into multiple ones such as three optical beams. The multiple optical beams will be processed to obtain FE, TE, and RF_SUM signal. The photo detection IC 16 is exemplified with four detectors (quad detectors), it should be noted detectors of 8, 12 or other numbers could be used for the photo detection IC 16.
The recording layer of write-once disk and re-write disk is generally a dye layer. FIG. 4 shows a prior art recording circuit for optical disk. FIG. 5A shows a writing signal produced by the recording circuit shown in FIG. 4; FIG. 5B shows a reflection coefficient resulted from the writing signal; and FIG. 5C shows a detection result of the photo detection IC 16 with respect to the writing signal shown in FIG. 5A.
As shown in FIG. 4 and FIG. 5A, an encoder 22 is used produce the writing pulses and an ALPC (auto laser power control) 24 controls the laser power according to the writing pulses. The laser impinges on an optical disk 18 to write data thereon. As shown in FIG. 5A, a mask signal 30 with a higher writing power is used to write a logical one (1) data and a space signal 32 with a lower reading power is used to write a logical zero (0) data. The encoder 22 produces a series of mask signal 30 and space signal 32 according to the data to be recorded and the ALPC 24 controls laser power according to the series of mask signal 30 and space signal 32.
As shown in FIG. 5B, the reflection coefficient of the disk is reduced by the higher writing power. The laser power does not instantaneously reach high level and the reflection coefficient of the disk has gradual change as shown in FIG. 5B. Therefore, the detection result of the photo detection IC 16 has waveform shown in FIG. 5C.
FIG. 6 shows a prior art photo detection IC 16 with sample/hold (S/H) circuit 20. The S/H circuit 20 is incorporated to each channel of the photo detection IC 16 to prevent jitter in output signal of the photo detection IC 16. The S/H circuit 20 generally works quite well in low speed recording operation of the recording apparatus for optical data.
The operation voltage of analog front end (the circuit portion connected to the photo detection IC 16) is now reduced to about 3.3V due to the progress in semiconductor manufacture. However, the operation voltage of the photo detection IC 16 is still maintained at 5V. In high speed recording operation of the recording apparatus for optical data, the prior art recording apparatus for optical data may have problem because the photo detection IC 16 output a higher voltage (such as 4V) than the operation voltage of analog front end. FIGS. 7A and 7B show switch circuit connected before the S/H circuit 20. The NOT gate 36 shown in FIG. 7A is implemented by PMOS M3 and NMOS M4 shown in FIG. 7B. It should be noted the output V1 is always smaller than the AFE (analog front end) voltage VCC.
When the control voltage V1 is at high state, the PMOS M1 is at low voltage and the NMOS M2 is at high voltage. When the control voltage V1 is at low state, the PMOS M1 is at high voltage and the NMOS M2 is at low voltage. The control voltage V1 is smaller than the AFE voltage VCC because those MOS transistors are powered by AFE voltage VCC. If VCC is 3.3V, then the control voltage V1 must be smaller than 3.3V. The PMOS transistor is turned on when gate-source voltage Vgs is larger than a threshold voltage Vth. The PMOS transistor is turned off when gate-source voltage Vgs is smaller than the threshold voltage Vth. The control voltage V1 must be smaller than the AFE voltage VCC and the output voltage of the photo detection IC is larger than the AFE voltage VCC. Therefore, the PMOS transistor is always turned on and has leakage problem. The pike 34 shown in FIG. 5C will cause error of S/H circuit in space signal region, especially in high speed operation.